Conventional ultrasonic diagnostic apparatuses irradiate a subject with an ultrasonic wave and convert the intensity of a reflected echo signal thereof into the luminance of a corresponding pixel, thereby obtaining a tomographic image of the structure of the subject. Further, in recent years, it is attempted to obtain an elastic modulus of a subject based on the movement of the subject measured precisely by analyzing the phase of a reflected echo signal.
For example, JP 10(1998)-5226 A describes a method of tracing a tissue with high accuracy by determining momentary positions of a subject using both the amplitude and the phase of a demodulation output signal of a reflected echo signal, thereby capturing micro vibrations on a large amplitude displacement motion caused by heartbeats. The method of tracing a tissue of a subject described in JP 10(1998)-5226 A will be described with reference to FIG. 21.
In FIG. 21, y(t) and y(t+ΔT) represent received signals of ultrasonic pulses transmitted at an interval of ΔT in the same direction of a subject. Herein, t represents a time. Assuming that the pulse transmission time is t=0, the reception time t1 of a received signal from a certain depth x1 is expressed as follows: t1=x1/(C/2), where C represents a sound velocity. Assuming that the phase displacement between y(t1) and y(t1+ΔT) is represented by Δθ, and the center frequency of the ultrasonic wave in the vicinity of the time t1 is represented by f, the movement Δx of x1 during the period ΔT is expressed as follows:Δx=−C·Δθ/4πf  (1)By adding Δx to x1, the position x1′ after ΔT second can be obtained as follows:x1′=x1′+Δx  (2)By repeating this operation, it is possible to trace the same region x1 of the subject.
Further, as an example of an advanced form of the method described in JP 10(1998)-5226 A, JP 2000-229078 A describes a method of obtaining a local elastic modulus by precisely tracing a large amplitude displacement motion caused by heartbeats on each of an inner surface and an outer surface of a blood vessel wall. According to this method, the motion velocity of micro vibrations superimposed on a large amplitude displacement motion is obtained, a strain of a blood vessel wall is measured based on a difference in velocity, and a local elastic modulus is obtained based on the strain and a difference in blood pressure. This method also makes it possible to display a space distribution of the elastic modulus visually. The method of calculating an elastic modulus described in JP 2000-229078 A will be described with reference to FIGS. 22A and 22B.
FIG. 22A shows a blood vessel 300 with an atheroma 303 by way of example. A probe 101 irradiates a subject 304 with an ultrasonic wave and receives an echo from the blood vessel 300, particularly an artery. Measurement points A and B are set on a blood vessel wall, and received signals from the measurement points A and B are analyzed by the above-mentioned method, whereby the movement (position) of each of the measurement points A and B is traced.
As shown by an ECG waveform in FIG. 22B, the artery contracts and expands repeatedly in response to heartbeats. Accordingly, the movements of the measurement points A and B are periodic as shown by tracing waveforms TA and TB, respectively. That is to say, the movements of the measurement points A and B follow the movement of the blood vessel wall that expands rapidly when the heart contracts and contracts gradually when the heart expands. A waveform W(=TB−TA) showing a change in the thickness between the measurement points A and B can be obtained from the tracing waveforms TA and TB. Assuming that the change amount of the thickness change waveform W is represented by ΔW, and the reference thickness when the measurement points are initialized is represented by Ws, the strain ε between the measurement points A and B is expressed as follows:ε=ΔW/Ws  (3)Assuming that the difference in blood pressure at this time is represented by ΔP, the elastic modulus Er between the measurement points A and B can be obtained by the following equation:
                                                        Er              =                              Δ                ⁢                                                                  ⁢                                  P                  /                  ɛ                                                                                                        =                              Δ                ⁢                                                                  ⁢                                  P                  ·                                      Ws                    /                    Δ                                                  ⁢                                                                  ⁢                W                                                                        (        4        )            By performing this operation with respect to a plurality of points on a tomographic image, a distribution image of the elastic modulus can be obtained.
However, in the method of tracing a tissue described in JP 10(1998)-5226 A, since a position change is added sequentially, errors that have occurred due to various causes such as noise and hand movement are accumulated, resulting in a decrease in tracing accuracy. In the method described in JP 2000-229078 A, in order to solve the above-mentioned problem, the tracing position is initialized regularly by an R wave detected from an electrocardiogram. However, in order to carry out an electrocardiogram, it is required to attach electrodes to at least three places of a subject and it takes time and labor to attach/detach the electrodes.